This invention relates generally to pulp bleaching and more particularly to a method for economically operating an ozone generator in connection with such pulp bleaching.
In the ozone bleaching process, ozone gas is generated on site either from air or from oxygen gas. Typically between about one percent and seven percent by weight ozone is discharged from an ozone generator in a carrier gas of oxygen or air, depending upon the feed gas of the ozone generator. This mixture is fed to an ozone reactor in which it is brought into contact with wood pulp for bleaching the pulp. In view of the reaction kinetics between ozone and pulp, a high concentration of ozone in the carrier gas is desired in order to maintain a high reaction rate and to minimize the contact time required between the pulp and the ozone gas. Since ozone concentration from an ozone generator is directly controlled by oxygen concentration in the feed gas, it is clear that oxygen rich feed gas is desirable for the ozone generator. Such oxygen rich feed gas can be provided as essentially pure oxygen from distillation of air, or as oxygen enriched air resulting from adsorption of nitrogen from air which is passed through a pressure swing adsorption unit. In any case, even when relatively pure oxygen is used as the feed gas for the ozone generator, usually only about four to seven percent ozone results. Therefore, a large fraction of the gas in an ozone/pulp contacting reactor consists of relatively pure oxygen carrier gas. After reaction with the pulp, this gas is discharged from the reactor as spent gas together with byproduct gases such as carbon monoxide, carbon dioxide, various hydrocarbon molecules, water vapor, and a minimal amount of residual ozone. The spent gas is usually passed through a hydrocarbon destructor and a gas dryer in order to return relatively pure gas to the ozone generator as feed gas. This process, however, permits a gradual build-up of carbon dioxide in the system due to generation of carbon dioxide as a byproduct of the pulp/ozone reaction and as a result of catalytic oxidation of carbon monoxide and hydrocarbons in the hydrocarbon destructor.
Although carbon dioxide is not harmful to the ozone/pulp reaction, the accumulation of carbon dioxide in the recycled oxygen gas stream reduces the concentration of oxygen in the feed gas and, consequently, ozone concentration in the reagent gas being fed to the bleaching reactor. With each, cycle through the system, the percentage of carbon dioxide increases, thereby increasing specific power consumption and further retarding the reaction. In order to alleviate this problem, a part of the spent gas is purged from the gas loop so that, when replaced with pure oxygen or enriched air, the carbon dioxide content of the carrier gas can be maintained at an acceptable level. This level is usually determined based on the price of oxygen, electrical power cost, and required concentration of ozone. Purging of 10-20% of the gas volume is normally necessary to maintain the concentration of oxygen within the acceptable range. For example, equilibrium oxygen concentration in the recycled spent gas is around 80-85%, when about 10% of the spent gas is purged, and cryogenically produced oxygen at virtually 100% purity is used as makeup oxygen.
When using oxygen enriched air produced by a pressure swing adsorption generator, about 90-95% of the feed gas to the ozone generator is oxygen. The balance of the feed gas is nitrogen which has not been adsorbed and argon or other gases which are not adsorbable in the zeolite bed of the reactor. Thus, in addition to the carbon dioxide, argon and other essentially inert gases (with respect to pulp) also accumulate in the gas loop. This necessitates increased purging of gas from the system and a resulting increased demand for makeup gas. In any case, control is required in order to provide the proper level of purging and makeup gas additions. In some cases, a single pass system is used, in which spent gas from the ozone pulp reactor is discarded or is reused for purposes unrelated to ozone generation. In any case, an economic cost is imposed on the operation.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.